Track Page progress with StateFlow (#8749)

* Update ReaderProgressIndicator documentation

ReaderProgressIndicator is not always determinate (cc554530, #5605).

* Track Page progress with StateFlow

(cherry picked from commit 593172f891)

# Conflicts:
#	app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/pager/PagerPageHolder.kt

source-api/src/main/java/eu/kanade/tachiyomi/source/model/Page.kt

@@ -2,6 +2,8 @@ package eu.kanade.tachiyomi.source.model
 
 import android.net.Uri
 import eu.kanade.tachiyomi.network.ProgressListener
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.Transient
 import rx.subjects.Subject
@@ -27,8 +29,14 @@ open class Page(
         }
 
     @Transient
-    @Volatile
-    var progress: Int = 0
+    private val _progressFlow = MutableStateFlow(0)
+    @Transient
+    val progressFlow = _progressFlow.asStateFlow()
+    var progress: Int
+        get() = _progressFlow.value
+        set(value) {
+            _progressFlow.value = value
+        }
 
     @Transient
     var statusSubject: Subject<State, State>? = null